Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing

Brown, Nicolas; Galbraith, Eric D.

doi:https://doi.org/10.5194/cp-12-1663-2016

Articles | Volume 12, issue 8

https://doi.org/10.5194/cp-12-1663-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-12-1663-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 8

Research article

|

18 Aug 2016

Research article |

| 18 Aug 2016

Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing

Nicolas Brown and Eric D. Galbraith

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Final revised paper (published on 18 Aug 2016)
Supplement to the final revised paper
Preprint (discussion started on 05 Oct 2015)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC C2009: 'lack of justification of proposed mechanism', Olivier Arzel, 14 Oct 2015
- AC C2778: 'Response to comment by Olivier Arzel', Nicolas Brown, 29 Dec 2015
RC C2155: 'Comments on 'Hosed vs. Unhosed: Global response to interruptions of the Atlantic Meridional Overturning, with and without freshwater forcing'', Anonymous Referee #1, 30 Oct 2015
- AC C2784: 'Reply to comment by Referee #1', Nicolas Brown, 29 Dec 2015
RC C2360: 'Review of “Hosed vs. unhosed: global response to interruptions of the Atlantic Meridional Overturning, with and without freshwater forcing” by N. Brown and E. D. Galbraith', Anonymous Referee #2, 18 Nov 2015
- AC C2787: 'Reply to comment by Referee #2', Nicolas Brown, 29 Dec 2015

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (21 Jan 2016) by Gerrit Lohmann

The experiments suggest that the large-scale responses of climate and ocean biogeochemistry to an AMOC disruption are quite similar regardless of whether the interruption is forced through hosing or as a spontaneous result of internal model dynamics.
This result is interesting. However, Referees #1 and #2 states that the un-hosed oscillations are not explored in detail.
To give an example, Referee #1 said that we need more analyses to support the complexity of the dynamics, where I agree.
Is the dynamics really like a type of “salt oscillator”?
You shall convince the reader about the mechanism in your model.
Please check also the wording of states. Is an oscillation necessarily related to multiple states?

Another example is related to "Apart from the small amplification of the general trends, we note one distinct additional feature: the Antarctic temperature response is decreased under hosing, weakening the bipolar seesaw. This feature of the Antarctic response appears to reflect the transport of freshwater from the North Atlantic to the Southern Ocean through relatively shallow pathways in the Atlantic, so that it freshens the Southern Ocean surface." I think it is necessary to make more detailed analyses that "The addition of freshwater strengthens the Southern Ocean halocline, reducing ocean heat release and keeping a larger mantle of sea ice around Antarctica. Thus, the cooler Southern Ocean does not reflect a different response to the AMOC weakening itself, but rather a secondary effect of the freshwater addition through its direct influence on the vertical density structure of the Southern Ocean."
This can be explored in detail (may be partly supplement). Can this finding be reconciled with paleo records? Just comment.

Please revise the paper according to the referees' comments.

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AR by Nicolas Brown on behalf of the Authors (19 Mar 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Apr 2016) by Gerrit Lohmann

RR by Anonymous Referee #3 (09 May 2016)

Suggestions for revision or reasons for rejection

This is an interesting and well-written paper worth publishing in Climate of the Past.
It documents interesting AMOC oscillations that occur without external freshwater input.
I recommend publication after minor revisions. Please find a few comments below.

Please add a reference to figure 5 in paragraph L. 186-199.
AMOC changes could be added to figure 5 to make the link more straightforward.

L. 269: I don't think that a southward shift of the ITCZ is a direct consequence of sea ice expansion during stadials, but it is due to the meridional temperature change. There is a lot of literature on southward ITCZ shift, such as Broccoli et al. 2006 (GRL), Stouffer et al. 2006 (j. Climate), that could be added here.

L. 279-282: The precipitation response in the western part of the Equatorial Pacific varies also significantly in the 3 cases: PI hosing, glacial hosing and unhosed. I understand that a detailed study of the reasons behind these changes cannot be done here. But, is this due to the different responses of the PMOC as well as Southern Ocean ventilation?

L. 288: Export production changes have also been studied in Obata 2007 and Menviel et al. 2008.

L.307: I would rather use “Arabian Sea” than “Pakistani margin”.

L. 310: I am not sure that the apparent mismatch between model and proxy off Baja California is due to a lack of coastal upwelling in the model. The main upwelling regions, including Baja California should be represented to some extent in the model. However, since export production changes off Baja California will depend on the upwelling strength and thus on the wind changes, the ENSO response will most likely have a strong impact. In addition, PMOC changes might also modulate the strength of the trade winds in the area and thus the upwelling off Baja.

3.4. Paragraph: Hosing the unhosed
Little additional information is obtained from this paragraph and figure 11. Figure 11 could be more informative if the unhosed panels were put in a second column next to the “hosed-unhosed” to have an idea about the magnitude of the changes in the “hosed-unhosed” compared to unhosed. In addition, it should be noted that the AMOC is even weaker in the hosed than in the unhosed. So of course, it is expected that the hosed will have a stronger response.

Typos:
L. 260: "rather than"

L. 384: “ in a relatively weak state”

References L. 720-721: lots of weird characters showing up in my version…

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RR by Anonymous Referee #1 (09 May 2016)

ED: Publish subject to minor revisions (review by Editor) (20 Jun 2016) by Gerrit Lohmann

AR by Nicolas Brown on behalf of the Authors (08 Jul 2016) Author's response Manuscript

ED: Publish subject to technical corrections (12 Jul 2016) by Gerrit Lohmann

AR by Nicolas Brown on behalf of the Authors (20 Jul 2016) Manuscript

Short summary

An Earth system model is used to explore variability in the global impacts of AMOC disruptions. The model exhibits spontaneous AMOC oscillations under particular boundary conditions, which we compare with freshwater-forced disruptions. We find that the global impacts are similar whether the AMOC disruptions are spontaneous or forced. Freshwater forcing generally amplifies the global impacts, with tropical precipitation and the stability of polar haloclines showing particular sensitivity.